I've never used the midi channels on my KBs, except for now using one for the Pro12 tablet which works beautifully with the Songbook on my KORG Pa900 ...
The onboard VH works very well (as Donny attests to above) but for the 50's gig I will be doing in NJ this summer, I am thinking of using my Digitech Vocalist Workstation instead of the onboard VH so that I can have a separate line out for the vocals - I know the sound guy they use for this gig would prefer that over just having the vocals come out the same mix as the KB - my question is "how can I use the midi channels to take just the vocal, put it through the Digitech, have it create harmony based on my left hand (or right hand as suggested by UD) chords, and still use a midi channel for the Pro12 tablet" ... this is probably answered in the User Manual, but I am hoping to get simple step by step directions in functional English here ...

All the keyboard will transport via midi, is midi. No audio. You'll have to plug into the harmonizer for that. But I think you already know that.
You will probably need to be sure the channel transmitting chord or note info from the keyboard matches the input channel of the harmonizer. Most are fairly automatic these days, but you may have to set whether the kb transmits below or above the split, or both.
This setting is generally made via instructions from the harmonizer, but touching a key below or above the split.
Also, be certain the harmonizer is set to not receive any program change or control change information, or else be sure the keyboard is not transmitting it. It will make changes to your effects, and maybe other things you don't want to happen.
Again, with the VK and the Harmony M, you can make this setting from the harmonizer quite easily. You'll have to read the manual on yours to see how to do it.
You will need to use a midi Through on either the harmonizer or tablet to use both from the same connection. I don't recall whether the PA900 has two midi outs, but I think it does not. There may also be a midi merge or split device available but I'm not familiar with those so can't help there.